1. Field of the Invention
The present invention relates to a vibration motor, and in particular to a vibration motor to be used for providing a physical sensation in a portable communication device such as a mobile phone and a PHS (Personal Handy-phone System) and a game device.
2. Description of the Related Art
For notifying a user of an incoming call in communication devices such as a mobile phone, two methods are available. In one method, sound is generated at the time of receiving the incoming call to acoustically inform the user of the incoming call, and in the other method, vibration is generated by a built-in vibration motor to physically inform the user of the incoming call. These two methods are both incorporated in the communication devices and can be switched over as appropriate.
Also, in order to enhance entertaining property in game devices, some game devices transfer vibration generated in steps with progresses of games by vibration motors embedded in the game devices to operators of the game devices.
A vibration motor of this type is configured to be secured on a circuit board in the device and, vibration is generated, for example, by a rotating eccentric weight attached to a motor shaft and the generated vibration is transferred to a housing of the device.
A vibration motor is mounted in a small-sized device such as a portable communication device in many cases and, therefore, miniaturization of the vibration motor itself is required. Conventionally, an eccentric weight is attached to an end of a motor shaft to generate vibration. Such configuration presents a problem that the eccentric weight is mounted outside a motor frame and, therefore, when the vibration motor is embedded in the portable communication device or the like, it is necessary to form an appropriate gap in a portion around the eccentric weight, which, as a result, causes complicated design.
To solve the above problem, in Patent Reference 1 (Japanese Patent Application Laid-open No. Hei 11-289734) or Patent Reference 2 (Japanese Patent Application Laid-open No. Hei 8-256449), inventions are disclosed in which, in order to be able to generate vibration without mounting the eccentric weight outside the motor frame, one of three salient poles being a magnetic pole of an armature core is removed and the remaining two salient poles are made to have eccentricity to generate vibration.
However, in the configuration as disclosed in Patent References 1 and 2 in which one of the three salient poles is removed, there is a problem that a cogging phenomenon increases and actually the motor would not rotate or sufficient output from the motor can not be obtained.